1. Field of the Invention
This invention relates to a method of and an apparatus for a getter-containing vacuum tube. More specifically, the invention concerns a method of manufacturing a getter-containing vacuum tube, which permits reduction of the time of manufacture. The invention can be utilized as a method of manufacturing cathode-ray tube (hereinafter also referred to as CRT) to be used for various purposes.
2. Related Arts
The getter has been used in the CRT for adsorbing gases in the tube.
In the usual manufacturing process, a getter (many getters being mainly composed of Ba), which is mounted inside the tube, is heated to about 800.degree. C. or above by externally applied high frequency power for flashing.
The general technique concerning the getter-containing vacuum tube will now be described with reference to FIGS. 4 and 5.
As shown in FIG. 4, inside a CRT 1 a getter 2 is supported by a getter spring 12 which is mounted on the inner surface of the back of a tube 10. Designated at 11 is an anode button, and at 13 a projection. Designated at 14 is a screen formed on the inner surface of the front face of the tube 10. On the screen 14, an image is formed by electrons shot from an electron gun 15. Designated at 16 is a conductive layer.
FIGS. 5A to 5C illustrate the principles, under which the getter 2 captures gases inside the CRT 1 to maintain a degree of vacuum. Specifically, heating means 3 (usually a high frequency induction heating coil) is provided outside the CRT 1 (FIG. 5A). The getter 2 is heated by the heating means 3 to 700.degree. to 1,200.degree. C. in case when the getter 2 is mainly composed of Ba, thus causing spattering and deposition of Ba (FIG. 5B). In this way, a Ba film 21 is formed on the inner wall surface of the tube. When the gas is spattered, inner gases are absorbed. More specifically, Ba absorbs oxygen and nitrogen (and also gases composed of these elements) in the CRT 1 in the form of oxides and nitrides. For example, it reacts with and absorbs O.sub.2, CO, CO.sub.2, etc. generated during the duty operation of the CRT, thus maintaining a degree of vacuum. In addition, it has an effect of removing oxides of carbon having adverse effects on the cathode. (The oxides of carbon are thought to reduce the life of the cathode, for instance with conversion of CO.sub.2 into CH.sub.4).
What is used as the getter 2 is typically mainly composed of Ba. For example, it is possible to use a getter composition, which comprises BaAl.sub.4 and Ni in a part-by-weight ratio of 50:50 and contains, if necessary, 0 to 4.8 parts by weight of Fe.sub.4 N. When this getter is heated to about 800.degree. C., a reaction represented as EQU BaAl.sub.4 +4Ni=&gt;Ba+4NiAl
is brought about, whereby spattered Ba has an effect of absorbing gases to maintain a degree of vacuum.
To capture gases by spattering the getter in the above way is referred to as getter flash. In the process of the CRT manufacture, it is desired to reduce the time required for the getter flash process.
In order to reduce the process time, it is important how to reduce the heating time until a sufficient amount of spatter is secured after the start of the getter flash.
As noted above, in the usual technique the getter mounted inside the CRT is externally heated by a high frequency coil to about 800.degree. C. to cause reaction between Ni and Al so as to cause Ba to flash. When flashed, Ba has an effect of absorbing gases generated in the CRT to maintain a vacuum degree thereof. It is thought that the life of the CRT is longer the greater the amount of Ba.
Investigations conducted by the inventor reveal a certain period of time is necessary to secure a predetermined spattering amount of getter, i.e., Ba. FIG. 6 shows a temperature rise curve I' in a getter flash process. In the graph, the ordinate is taken for temperature, and the abscissa for time. Indicated by ST on the abscissa is the flash start time, and by TT the getter (i.e. Ba) spattering securement time. It was found by the inventor that the getter spattering securement time TT is a function of the flash start time ST.
It is thought to be possible to reduce the getter spattering securement time TT by reducing the flash start time ST. To reduce the flash start time ST, however, it is necessary to increase the RF output of a high frequency heating coil as heating means. The getter is usually held in a SUS or like container. Therefore, by suddenly increasing temperature for reducing the flash start time ST, a melt temperature, at which the container is melted (i.e., 1,400.degree. C. in SUS) is reached before the reaching of the getter spattering securement time TT, as labeled at V in FIG. 6. That is, the container is melted.
Generally, the heating temperature and the high frequency (RF) output are related as shown by curve A in FIG. 7, the heating temperature being increased in proportion to the high frequency output.
It was found as a result of investigations conducted by the inventor that by carrying out the heating for long time with the high frequency (i.e., RF) output held constant, a peak is produced at a certain predetermined temperature, as shown by curve B in FIG. 7.
The relation between the container melt and the temperature is as shown by curve C in FIG. 9. As shown, the container which comprises SUS is melted when the temperature is increased beyond 1,400.degree. C.
Specifically, as shown in FIG. 10, by increasing the RF output as shown by curve IV for reducing the process time with respect to the usual output RF temperature increase curve as shown by curve III, the melt temperature is reached to result in melting within the getter spattering securement time TT.